PSTN network topology
[1] The ideas were first developed in the Bell System in the United States, but were soon adopted by other countries where telephone companies were facing similar issues, even when servicing smaller geographic areas.
While the following discussion refers to AT&T and (principally) to the United States, it is important to remember that until 1975, AT&T controlled Bell Canada and thus influenced corporate decisions north of the border.
By the mid-1920s, a revised manual system where "local" toll operators connected tandem routes (a process formally called Combined Line and Recording) as needed to complete telephone calls, reduced the process to an average of two minutes, but still meant that some complex routing might interconnect as many as sixteen points.
As long-distance services grew in the Contiguous Continental US (48 states) and Canada, the amount of overhead equipment and people required to determine and establish Rates and Routes became excessive.
These areas were handled as International Calls until more advanced computer hardware and software allowed them to be included in the automated, integrated systems in later decades.
There were twelve Regional Centers in North America, ten in the United States, nine of which were operated by AT&T (White Plains, NY, Wayne, PA, Pittsburgh, PA, Norway, IL [a rural crossroads west of Chicago at the intersection of US highway 52 and IL highway 71 - an underground office built with hardened construction to withstand nuclear attack], Conyers, GA in Rockdale County, St Louis, MO, Dallas, TX, Denver, CO, Sacramento, CA), and one in a GTE service area (San Bernardino, CA).
The realignment and dispersion of functions were done, in part, to ensure maximum network integrity in the event of a national emergency, a major concern in that era.
The toll center is also used to connect to the long-distance network for calls where added costs are incurred, such as operator handled services.
The breakup of the Bell System, and the need for each of the surviving regional operating companies to handle long-distance interconnections, also promoted the inclusion of inter-regional and international processing through larger Class 4 offices.
The technology to automate these connections through "regular" operator traffic positions began to develop in the 1960s (see Bell Laboratories Record 42:7, July–August 1964).
As the decade of the 1970s progressed, North American customers who were served by electronic offices began to be able to directly dial to an increasing number of international points, a service known as IDDD (International Direct Distance Dialing), (service between ESS offices in New York and London began on March 1, 1970).
The forerunner of British Telecom, the General Post Office, also organized its intercity trunk network along similar hierarchical lines to that of North America.
However, because of the significantly smaller geographic area involved, fewer levels of connection were required, and no formal numbering of class offices was made.
There were a few special exceptions to the following description, notably those involving Northern Ireland, some of the Channel Dependencies, and the few locations in England which were served by non-GPO companies, such as Hull (KCOM Group) and Portsmouth.
[citation needed] In the early days of manual exchanges, outlying areas (eventually called dependent exchanges) were connected through progressively larger locations (eventually called group switching centres) into one of the main cities - Birmingham, Edinburgh, Glasgow, Liverpool, London, and Manchester.
There were also some additional Local Tandems to handle traffic in the London Metropolitan Area without involving the GSCs, although this was a later development, as it required common control signalling for identification.
With the advent of subscriber dialed calls, numbering patterns were reassigned to provide for mnemonic methods of improving customer performance.
However, because of subscriber dialing errors, there was an early decision to eliminate codes that began with 00 and Oxford soon became 0865, the sequence 86 designating the first two letter of university.
As Strowger switches were retired and replaced with electronic systems, Subscriber Trunk Dialing codes no longer followed the original rules, and were significantly revised in the mid-1990s, with further changes as wider use of mobile phones and non-BT competition came into the UK market.
Repair of much of the network had been deferred during the war due to lack of parts, as well as co-opting of technical personnel for German military needs.
Also, during the 1970s and 1980s, the smaller rural switches were replaced and combined with nearby automated offices, and a closed numbering scheme was adopted for dialing consistency.
In addition, there are 12 CTI (centre de transit internationaux) for connections to areas which are not integrated into the French telephone network [note that some overseas locations are considered "domestic" for telecommunications purposes].
